Project information

Project financing:

Deutsche Forschungsgemeinschaft
Hanns-Seidel-Stiftung

Abstract

This work aimed to elucidate the structures of disordered or microcrystalline benzene-1,3,5-trisamides (BTAs) using NMR crystallography and total X-ray scattering in order to understand the role of dipole moments in extended supramolecular crystals.
Selected sample BTAs, which assemble into columnar stacks via triple helical hydrogen bond formation, resulting in large macrodipole moments along the columnar axis, showed pronounced disorder manifesting in diffuse X-ray scattering from single crystals. The disorder is caused by geometric frustration of the macrodipole interactions of neighbouring columns due to their arrangement in hexagonal rod packings. The disorder could in all cases be explained by Monte-Carlo simulations of two-dimensional Ising antiferromagnet-like models. Reduction of the macrodipole interactions by chemical design of the molecules - namely, the choice of long and bulky side groups enhancing the intercolumnar distance or the introduction of polar C-F bonds via substition of methyl groups by fluorine reducing the macrodipole size - tuned packing effects, which introduced ferroelectric next-nerarest neighbour interactions. In this way, the creation of axially polar domains with spontaneous, permanent polarisation was facilitated in these columnar phases solely by targeted design of the molecular synthons.
The introduction of fluorine not only enabled a tuning of the macrodipoles but also resulted in polymorphism. Beside the columnar structure established by single-crystal diffraction, a second polymorph forming only microcrystalline powders was found by solid-state NMR. Real-space structure solution from powder X-ray diffraction data yielded a reasonable model but did not allow for a differentiation of fluorine atoms and methyl groups due to their equal electron number. Therefore, 19F double-quantum experiments using homonuclear dipolar recoupling in the presence of a dense 1H spin network were implemented. This allowed to distinguish different structure models differing only in the fluorine-methyl ordering and to show that the fluorine atoms are ordered due to a weak NH-F interaction. The structure solution showed that a weak NH-F interaction leads to ordering of the fluorine atoms and that a two-dimensional hydrogen bond patttern results in sheet-like aggregates, which only interact via van der Waals forces, leading to a highly anisotropic structure. The reason for this unusual packing pattern of BTAs is assumed to lie in the reduced cooperativity in the self-assembly of columnar stacks because the polar C-F bonds reduce the macrodipole moments.
This two-dimensional hydrogen bond pattern was emulated by bisamides, bisureas and bisacylureas, where the molecular design allows an exact cancellation of dipole moments. Self-assembly yielded nanoscopic sheets with high aspect ratios; structure solution using NMR crystallographic approaches showed that all structures exhibit extended hydrogen bond patterns, forming either two-dimensional layers of molecules or one-dimensional chains, which pack densely to two-dimensional sheets, explaining the anisotropic self-assembly. The molecules always adopt a configuration which intrinsically cancels the dipole moments within the molecule, so that the self-assembly is purely hydrogen-bond driven.